In this type of boot, the rigid shell provides the most effective solution guaranteeing that the skier's lower leg will be held firmly in place and ensuring water-tightness, thermal insulation, and, above all, the almost entirely rigid, mechanical transmission of the points of support between the lower leg and the ski. On the other hand, this solution requires the use of internal tightening means which provide for adjustment to the morphology of the foot and its position-retention, these means being totally separate from the means for tightening and/or closing the upper over the lower leg. Moreover, because the foot has multiple joints, design of the internal tightening means also requires consideration of the volume enclosed in the boot when movement dynamics take place, while taking into account the variations of the foot between its static and load-bearing positions. These variations may measure approximately 12 millimeters in width, 5 millimeters in length, and 2 millimeters resulting from the increased volume of the malleoli. In fact, to ensure ideal support of the foot, the internal tightening means must provide the various support configurations required (seating; transverse position-retention, etc.), while allowing the indispensable degree of freedom necessary to be able to adopt without obstacle the extreme positions which the load-bearing foot may take during skiing. These degrees of freedom are also necessary so as not to impede blood circulation and/or nerve perceptions.
To solve these problems relating to differentiated tightening systems between the lower leg and the foot and to the position-retention of the foot while fulfilling the requirements governing the range of freedom in conventional boots of the type specified above, several structures of the upper have been proposed. As an example, mention may be made of the boots described in EP 479 123 and DE 19 63 342, and in EP 232 218 and FR 2 498 431.
In the example given by patent application No. DE 19 63 342, the ski boot has a shell open longitudinally and in which the front part of the upper is formed from several successive elements jointed together beginning at a boot end-piece which is, in turn, jointed to the shell base. According to this disclosure, foot-tightening and position-maintenance are obtained by means of a tightening device which presses the boot end-piece on a lining made of a honeycomb material inserted between this end-piece and the top of the foot, at the same time that this device acts on the water-tight tongues which delimit the longitudinal opening. As regards position-maintenance and tightening of the upper over the lower leg, a strap encloses the upper element of the front portion of the upper, which in fact forms a cuff, and the rear cover is positioned on the wings emerging from the shell base. As is true for the foot, position-maintenance of the lower leg is produced by means of a honeycomb lining inserted between the lower leg and the upper components, i.e., the rear cover, the cuff, and the wings in the shell base.
By virtue of this structure of the front part of the boot upper, the tightening systems are clearly distinguished, but the foot is continuously compressed. Indeed, because the vertical motion of the jointed elements belonging to this front structure in relation to the top of the foot is not limited, it is the foot which forms the lower closing stop restricting the motion of these elements. Moreover, when the boot is closed and the foot becomes load-bearing, it is the foot which, when the size thereof varies during motion dynamics, must push these elements back so as to acquire the freedom of movement indispensable to it. Moreover, this upper structure does not guarantee a constant position of the front support configuration of the lower leg on the element forming a cuff from one boot to another and/or from one skier to another for a given size. In fact, since the lower closing stop of the front components composing the upper is the foot, a "strong" foot will position the cuff higher than does a "weak" foot. It will also be noted that the problem of impermeability between the cuff and the elements located on the foot is solved by a thin rubber lining, thereby complicating the manufacture of the front structure of the upper.
Again, because the cuff element is jointed in the area of articulation of the ankle to the intermediate element located substantially on the flexible bend area, the lower front edge of the cuff moves away from the upper edge of the intermediate element. Consequently, as soon as the element pivots on the front structure, it is no longer guided on the front and can become deformed transversely when the skier's lower leg becomes supported toward the front. This front structure of the upper thus does not guarantee virtually rigid, one-directional forward mechanical transmission.
In the example of the boot described in EP 232 218, the same disadvantage occurs, since the cuff is mounted in a stationary hinged configuration on the shell base in the area of articulation of the ankle, without being guided in the central part, which is simply connected to the shell base using a flexible covering element, whose function is to ensure the impermeability of the flexible bend/instep area, where an internal tightening mechanism is located. Accordingly, the stresses which could be transmitted transversely to the cuff cannot be counterbalanced otherwise than by joining the cuff to the shell base in the area of articulation of the ankle. It is clear that the greater the distance separating the area in which the skier's lower leg is supported on the cuff and the joint, the more the cuff risks becoming twisted or bent transversely.
As regards the position-maintenance of the lower leg, and thus the closing of the boot upper on the latter, another tightening mechanism connects the cuff belonging to the rear cover in the top part of the upper independently of the internal tightening system. In this boot, the front cover does not hold the foot in place internally, and use must be made of an internal tightening mechanism for that purpose.
For comparative purposes, the boot described in FR 2 498 431 has a front closing cover which itself ensures the position-maintenance of the foot using a tightening strap carried on the lower ends of the tabs fastening the cuff to the shell base. On the other hand, as in the boot in DE 19 63 342, downward travel of the front cover is not limited, and it thus rests continuously on the top of the foot, thereby generating a continuous pressure capable of altering blood flow or nerve perceptions. Moreover, because the front cover is located under the cuff which carries the tightening strap, any forward motion of this cuff simultaneously causes an increase in the tightening force applied by the strap on this cover, thereby generating a substantial increase of pressure on the foot.
In EP 479 123, the boot has a front closing cover fitted with internal foot-tightening means, and this cover is limited in its lower position on stops belonging to sliding lateral-connection devices, a rear vertical extension of the cover constituting the equivalent of a cuff. In this type of boot, the tightening systems, i.e., one for holding the foot and the other for holding the lower leg in place, are made separate, but, because the front cover, which forms one piece with the cuff, moves loosely within the limits dictated by the sliding lateral-connection devices, any forward flection of this cuff resulting from the support of the skier's lower leg causes variations in pressure on the skier's instep.